The present invention relates to a percussive down-the-hole hammer for rock drilling, and a one-way valve used therein.
A prior art drill bit for a down-the-hole hammer is disclosed in U.S. Pat. No. 6,062,322. The drill bit comprises an extended anvil portion on which a piston impacts repeatedly to advance the down-the-hole hammer through the rock. The piston is actuated by pressurized air that is conducted along a longitudinal central passage of the apparatus. That passage extends within a center of the piston and then is distributed alternately to upper and lower ends of the piston to reciprocate the piston.
It is necessary to provide a check valve in such a hammer in order to prevent the backflow of groundwater or debris during periods when the operating air is shut off. A conventional check valve, disclosed in U.S. Pat. No. 6,062,322, comprises a dart that is disposed within the central longitudinal passage and is biased upwardly against a seat by a metal coil spring. When the operating air is turned on, the air pressure pushes the open by compressing the spring. When the operating air is turned off, the spring pushes the dart closed. One disadvantage of such an arrangement is that the spring is susceptible to fatigue and corrosion and may eventually fail.
In WO 99/64711 a check valve is disclosed which comprises a rubber cylinder mounted on the outer circumference of the top sub (or xe2x80x9cbackheadxe2x80x9d). A shortcoming of such an arrangement is that the top sub is a wear item and occasionally needs to be replaced, requiring that the valve be replaced as well. Also, the air passing through the valve must be displace along the inside wall of the cylinder which complicates the delivery of the air as compared to an apparatus in which the air is conducted through a central passage of the hammer.
It would, therefore, be desirable to provide a down-the-hole percussive hammer wherein a check valve does not have to be replaced along with the top sub, and wherein operating air can be conducted along a central longitudinal passage upon exiting the top sub.
A first aspect of the present invention relates to a down-the-hole percussive hammer for rock drilling. The hammer comprises a generally cylindrical casing, and a drill bit disposed at a front end of the casing. A piston is mounted longitudinally behind the drill bit in the casing for reciprocation in a longitudinal direction for applying impacts to the drill bit during each forward stroke of the piston. A top sub is mounted in a rear portion of the casing and includes a central passage for supplying operating air for reciprocating the piston. The central passage includes a chamber disposed between front and rear ends of the top sub. A hollow feed tube is mounted within the central passage, with a segment of the feed tube disposed within the chamber. The segment includes longitudinally spaced air outlet and re-entry ports communicating with the chamber. The outlet port is disposed rearwardly of the re-entry port. A seal is disposed within the feed tube for sealing an interior of the feed tube between the outlet and re-entry ports. A check valve is provided for permitting a forward flow of operating fluid and preventing a rearward flow of fluid. The valve includes an elastically resilient sleeve extending around an outer circumference of the feed tube for blocking the outlet port from the chamber. The sleeve is elastically expandible away from the outlet port by pressurized operating air to open the outlet port and permit operating air to flow out of the outlet port and into the chamber and then into the re-entry port.
The invention also pertains to the feed tube per se.